Thermal processing furnaces are commonly used to perform a variety of semiconductor fabrication processes, including but not limited to oxidation, diffusion, annealing, and chemical vapor deposition (CVD). Most conventional thermal processing furnaces typically employ a processing chamber that is oriented either horizontally or vertically. Vertical thermal processing furnaces generate fewer particles during processing, which reduces substrate contamination, are readily automated, and require less floor space because of their relatively small footprint.
One common application of thermal processing furnaces is the growth of high-quality thin dielectric layers during integrated circuit manufacture to provide, among other uses, device isolation, transistor gate dielectrics and capacitor dielectrics. Dielectric layers of silicon dioxide grown by conventional wet or dry thermal oxidation processes exhibit reduced quality and long-term reliability as the thickness is reduced. Oxidation of silicon in an ambient containing nitrous oxide (N2O) is recognized as a means for improving the quality of silicon dioxide dielectric layers, as compared to those grown via conventional dry or wet oxidation processes. The increased reliability of such dielectric layers may result from the incorporation of nitrogen atoms originating from the nitrous oxide into the silicon dioxide matrix to form a silicon oxynitride. After a thin layer of silicon oxynitride forms on the surface, the diffusion of oxidant species to the underlying silicon is greatly hampered. Thus, the resultant dielectric layers grown using a process gas containing nitrous oxide are thin. Other benefits accrue from the use of silicon oxynitride as a dielectric, such as superior diffusion barrier properties for certain common dopant species like boron used in semiconductor device fabrication.
When using nitrous oxide as a process gas for forming silicon oxynitride dielectrics, a problem that is encountered is process matching or the repeatability of a process among different tools when exactly the same process and hardware configuration is used. The outcome is different process results (e.g., thickness for the oxynitride layer and the nitrogen profile in the oxynitride layer) among different thermal processing furnaces. To increase throughput, a typical process line includes multiple thermal processing furnaces. Achieving process uniformity among the different thermal processing furnaces is desired but, unfortunately, rarely achieved.
One attribute of conventional hardware configurations is that the flow of nitrous oxide to the process chamber is regulated or metered by a mass flow controller. The delivery line transporting the metered nitrous oxide flow is divided at a common point in the gas manifold downstream from the mass flow controller. The divided metered nitrous oxide flow is conveyed to a pair of gas injectors, which inject the nitrous oxide process gas inside the process chamber for forming silicon oxynitride layers on the substrates held inside the process chamber. Upon injection, the nitrous oxide spontaneously undergoes an exothermic reaction that decomposes or cracks the nitrous oxide molecules to form nitric oxide (NO) and other reaction by-products (e.g., O2, N2), which react with the substrates to grow the oxynitride layer.
The gas injectors for thermal processing furnaces are hand manufactured from a length of dielectric tubing to specific dimensional tolerances that are predetermined according to the hardware configuration of the thermal processing furnace. Minor deviations from targeted manufacturing tolerances often result in a set of gas injectors that exhibits an asymmetrical or non-symmetric flow ratio among the individual gas injectors. Attempts to manufacture different sets of gas injectors to identical dimensional tolerances so as to exhibit identical non-symmetric flow ratios as a master set of gas injectors is difficult, if not impossible.
The flow ratio of the nitrous oxide injected into the process chamber is an important parameter in determining the reaction by-products of the cracking or decomposition. A variation in the flow rate ratio may cause a significant change in the nitrogen content and profile in the oxynitride layer and/or a significant change in the layer thickness. Because of the inability to manufacture dimensionally matched sets of gas injectors, significant differences in the properties of the oxynitride layer may be apparent for the same intended process executing on different conventional thermal processing furnaces, which is unacceptable.
There is thus a need for an improved apparatus and method for controlling the cracking of nitrous oxide in a thermal processing furnace that overcomes these and other disadvantages of the apparatus and methods currently used in conventional thermal processing furnaces.